KABUL — A man in an Afghan uniform shot and killed three American troops yesterday morning in southern Afghanistan, the US military command said, in the third attack on coalition forces by their Afghan counterparts in a week. The Taliban claimed the shooter joined the insurgency after the attack.

So far this year, at least 21 similar attacks — in which Afghan forces or insurgents disguised in Afghan uniforms have turned their guns on international troops — have killed 30 coalition service members, according to an Associated Press tally.

Yesterday’s shooting took place in the Sangin district of Helmand province, said US military spokeswoman Maj. Lori Hodge. She gave no further details and said the military were investigating.

The attack is the latest in a rising number of so-called “green-on-blue” attacks in which Afghan security forces or convincing imposters kill the US or NATO partners who are training Afghans to take over once most international forces leave in 2014.

The US is hoping the Afghan Local Police will be a key force to fight the insurgency after most international troops withdraw.